Often, it is necessary to measure the rate of fluid flow within a closed pipe. Non-invasive measurement methods are preferred because such methods do not detrimentally effect the fluid flow or pipe wall. There are two dominant methods of non-invasive flow rate measuring: xe2x80x9cDoppler ultrasonicxe2x80x9d and xe2x80x9cUltrasonic transit time.xe2x80x9d Both methods utilize the transmission of ultrasound through the pipe wall and into the fluid.
Doppler ultrasonic uses two ultrasonic transducers coupled to the pipe. The first transducer transmits a continuous ultrasonic signal through the pipe wall and into the fluid. Assuming the moving fluid contains bubbles or solids which can act as acoustic scattering sites, or xe2x80x9cscatterersxe2x80x9d, the second transducer receives scattered ultrasound signals. Then, the frequency of the scattered signal is compared with that of the transmitted signal. The frequency shift between the transmitted and received signals is proportional to the velocity of the scattering sites and, therefore, indicates the velocity of the fluid in the pipe.
In many industrial applications, vibrations from motors and other extraneous sources create frequencies in the pipe that are received by the ultrasonic sensor along with the Doppler shifted frequencies of interest. The detection system used to determine the Doppler frequency may select noise or other ambiguous signals, causing erroneous velocity measurements.
One past approach to dealing with the problem of extraneous frequencies involved the use of digital filters to mask out erroneous frequencies. This method assumes the noise source is stationary and continuous. This is often not the case in industrial applications. Variable frequency motor controls are now commonly used to control pumps, resulting in the presence of a range of noise frequencies some of which can get past the filters. Further, automated system controllers continuously turn noise generating equipment off and on, resulting in still further variation in the range of extraneous frequencies. Finally the digital filters can be complex to use.
There is a need in the art for a method and system for measuring flow rate with improved reliability and consistency which will not be subject to the confusion caused by extraneous frequencies being received by existing Doppler flow system transducers.
There is a another need in the art for a method and system which will accomplish such flow rate measurement without the need for customer installed filters.
There is yet another need in the art for a method and system for measuring flow rate which simplifies the user""s interaction with the required equipment.
There is yet another need in the art for a method of determining fluid flow rate in real time with maximum computational efficiency.
These and other objects, features and technical advantages are achieved by a system and method for determining the velocity of a flowing fluid by measuring the Doppler shift of two or more ultrasonic waves reflected from scatterers carried in a flowing fluid. This flowing fluid contains a dispersion of scatterers which comprise various bubbles and solids which reflect incident ultrasonic waves.
A series of ultrasonic waves is transmitted into the flow at a specific frequency and reflects off the flowing scatterers. The corresponding Doppler shifted reflection for this series of waves is subtracted from the transmitted waves and the result collected. A second series of waves at a different frequency is then transmitted into the flow and a second corresponding Doppler shifted reflection is subtracted from the transmitted waves and the result collected. For both series, the result of the subtraction contains Doppler beat frequencies representing the velocity of the fluid.
Performing a Discrete Fourier Transform (DFT) on the first subtracted series will reveal concentrations of energy at particular frequencies, some of which result from the scatterers and some of which result from extraneous noise. Similarly, a second DFT performed on the second subtracted series will also reveal frequency peaks representing both the Doppler beat frequencies and extraneous noise. The embedded noise and other extraneous signal peaks will be at similar frequencies in both subtracted signals while the Doppler beat frequencies will be separated by a difference proportional to the ratio of the transmitted frequencies.
This absence in variation of the noise frequencies between the two series of sampled waves permits the frequencies of interest to be isolated from the extraneous noise frequencies. The second collected series of frequencies is intentionally scaled by the ratio of the two transmission frequencies. This causes the Doppler beat frequencies to align and the noise frequencies to misalign. The xe2x80x9cscalingxe2x80x9d referred to above is performed on the second (and all additional sets if applicable) set of DFT data, and consists of the following: for each data point consisting of a frequency associated with an amplitude, multiplying the frequency datum by the ratio of the first transmission frequency to the second transmission frequency. This operation compensates for the inherent property of the Doppler effect which, for the same fluid and scatterer velocity, generates reflected velocity related Doppler beat frequencies (frequencies present after mixing) which are proportional to the frequency of the signal originally transmitted into the fluid.
A further improvement to the implementation of this method takes advantage of an inherent property of the DFT. By setting the sampling rate to be proportional to the transmission frequency for each series of data collected, the sampling time interval for the subtracted signal and the resulting frequency increment of the calculated DFT will be inversely proportional to the transmission frequency. Since the measured Doppler shifted beat frequency associated with a particular velocity is directly proportional to the transmission frequency employed, the sampling frequency adjustment mentioned above automatically compensates for the Doppler effect, thus obviating the need for mathematically scaling the frequency data after calculating the DFT. This represents an advantage because mathematically scaling the data is computationally expensive, and this invention concerns a real time process in which processing power must be optimally employed.
After sampling at the rate established for each transmission frequency and performing the DFT for two sets of data, the velocity related frequency data for the two data sets will converge within a reasonable frequency range without having to perform a separate calculation to mathematically scale the frequency values for the data collected at the lower transmission frequency. The scaling process effected by the variation in sampling rates will cause noise and other extraneous frequencies, which are the same between the two data sets prior to the sampling process, to misalign once the scaling is performed.
The aligned Doppler shifted peaks are detected by finding the maximum amplitude, or apex, of a correlation performed on the two sets of DFT data. A frequency distribution is identified as the Doppler distribution based on the location of this apex. Then, the centroid of the selected frequency distribution is determined and used as the measurement frequency, which is then used to calculate the velocity of the fluid. The fluid flow can then be determined by multiplying the cross-sectional area in the pipe by the fluid flow velocity.
It must be emphasized that although the above discussion has concentrated on the operations involved in determining fluid velocity using two transmission frequencies, the same principles can be readily applied for any number of additional transmission frequencies.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.